1. Technical Field
This invention relates to apparatus for vapor deposition of coatings in general, and to cathodic arc vapor deposition apparatus in particular.
2. Background Information
Vapor deposition as a means for applying a coating to a substrate is a known art that includes processes such as chemical vapor deposition, physical vapor deposition, and cathodic arc vapor deposition. Chemical vapor deposition involves introducing reactive gaseous elements into a deposition chamber containing one or more substrates to be coated. Physical vapor deposition involves providing a source material and a substrate to be coated in a evacuated deposition chamber. The source material is converted into vapor by an energy input, such as heating by resistive, inductive, or electron beam means.
Cathodic arc vapor deposition involves a source material and a substrate to be coated placed in an evacuated deposition chamber. The chamber contains only a relatively small amount of gas. The negative lead of a direct current (DC) power supply is attached to the source material (hereinafter referred to as the "cathode") and the positive lead is attached to an anodic member. An arc-initiating trigger, at or near the same electrical potential as the anode, contacts the cathode and subsequently moves away from the cathode. When the trigger is still in close proximity to the cathode, the difference in electrical potential between the trigger and the cathode causes an arc of electricity to extend therebetween. As the trigger moves further away, the arc jumps between the cathode and the anodic chamber. The exact point, or points, where an arc touches the surface of the cathode is referred to as a cathode spot. Absent a steering mechanism, a cathode spot will move randomly about the surface of the cathode.
The energy deposited by the arc at a cathode spot is intense; on the order of 10.sup.5 to 10.sup.7 amperes per square centimeter with a duration of a few to several microseconds. The intensity of the energy raises the local temperature of the cathode spot to approximately equal that of the boiling point of the cathode material (at the evacuated chamber pressure). As a result, cathode material at the cathode spot vaporizes into a plasma containing atoms, molecules, ions, electrons, and particles. Positively charged ions liberated from the cathode are attracted toward any object within the deposition chamber having a negative electrical potential relative to the positively charged ion. Some deposition processes maintain the substrate to be coated at the same electrical potential as the anode. Other processes use a biasing source to lower the potential of the substrate and thereby make the substrate relatively more attractive to the positively charged ions. In either case, the substrate becomes coated with the vaporized material liberated from the cathode.
The random movement of the arc can sometimes lead to nonuniform erosion of the cathode, which in turn can limit the useful life of the cathode. To avoid non-uniform erosion, it is known to steer the arc relative to the cathode. U.S. Pat. Nos. 4,673,477, 4,849,088, and 5,037,522 are examples of patents that disclose apparatus for steering an arc relative to a cathode. Some prior art steers the arc by mechanically manipulating a magnetic field source relative to the cathode. Other prior art steers the arc by alternately electrically connecting a power supply lead between two ends of a cathode. In both these approaches, the speed of the arc relative to the cathode is limited by the speed of the apparatus manipulating the magnetic field source, or switching the power supply. Another limitation is the complexity of the switching mechanisms and the hardware neccesary to manipulate a magnetic field source relative to the cathode. A person of skill in the art will recognize that a production coating environment is harsh and simplicity generally equates with reliability.
Presently available cathodic arc coaters typically use a cooled cathode fixed in place within the coater. One cooling scheme provides a manifold attached to the cathode that permits the passage of coolant between the cathode and manifold. Another scheme uses coolant piping connected to a hollow cathode. A problem with either scheme is that the cathode must be machined to accept the manifold or piping. Not all cathode materials are amenable to machining and even where possible, machining adds significantly to the cost of the consumable cathode. Another problem with "directly cooling" the cathode is the labor required to replace the cathode when its useful life has expired. In the previous example where a manifold (or piping) is mechanically attached to the cathode, the manifold (or piping) must be detached from the old cathode and attached to a new one, and the deposition chamber subsequently cleaned of coolant. For those applications which require cathode replacement after each coating run, the labor costs and down time can be considerable. Still another problem with direct cathode cooling is leakage. Coolant leakage occurring during deposition can contaminate the substrates being coated and require extensive cleaning within the deposition chamber. Airfoils for gas turbine engines are an example of an expensive substrate to be coated; one where it would be a distinct advantage to minimize or eliminate losses due to contamination.
In short, what is needed is an apparatus for cathodic arc vapor deposition of material on a substrate that operates efficiently, one capable of consistently providing a high quality coating on a substrate, one that optimizes cathode erosion, and one that operates cost effectively.